This study has been undertaken to identify capsaicin-sensitive nerve fibers in the sensory trigeminal system and to clarify the role of them in pain mechanisms. This study was composed of the following two experiments : (1) electrophysiological identification of A-, C-fibers in the infraorbital nerve (ION) and verification of their sensitivity to capsaicin, (2) electrophysiological identification of A-, C-fiber components field potential responses to electrical stimulation of ION in the nucleus spinal tract trigeminal nerve, subnucleus caudalis (Sp5C) and their relationship.These experiments provided the following results.The results obtained from the experiment (1) proved the insensitivity of A-fiber of ION to capsaicin and its reversible inhibitory effect on C-fiber of ION.The experiment (2) provided the following results. The region, where maximum field potential of C-fiber component responded to electrical stimulation of ION,was found in Sp5C.The reversible inhibitory effect of topical application of capsaicin to ION on the field potential of C-fiber component was verified. At the locus of maximum response of C-fiber, paired pulse test with interval change revealed that the inhibition of C-fiber activity with application of capsaicin to ION increased the field potential amplitude of A-fiber component response to the second stimulus at some intervals, suggesting that C-fiber input could modulate A-fiber input at the level of Sp5C.